The origins of the idiom, it’s raining cats and dogs, are uncertain. The most likely source is a satirical poem by Jonathan Swift, A Description of a City Shower, first published in Tatler magazine in 1710. The poem includes the lines “Drown’d Puppies, stinking Sprats, all drench’d in Mud / Dead Cats and Turnip-Tops come tumbling down the Flood.” There are more details here.